Thermodynamic design methods and performance calculation models for chemical reformers that can be used to recuperate exhaust heat and to improve combustion quality are investigated in this paper. The basic structure of the chemical reformer is defined as series-wound reforming units that consist of heat exchangers and cracking reactors. The CH4-steam reforming reaction is used in the chemical reformers and a universal model of this reaction is built based on the minimization of Gibbs free energy method. Comparative analyzes between the results of the calculation and a plasma-catalyzed CH4-steam reforming reaction experiment verify that this universal model is applicable and has high precision. Algorithms for simulation of series-wound reforming units are constructed and the complexity of the chemical reformers is studied. A design principle that shows the influence of structural complexity on the quantity of recovered heat and the composites of the reformed fuel can be followed for different application scenarios of chemical reformers.
Combustion
characteristics are significantly affected by ambient
pressure, which is usually much higher than the atmosphere in the
engine cylinder. In this work, the effects of pressure on the flame
structure and soot behavior in a methane diffusion flame doped with n-heptane were numerically investigated by a detailed chemical
mechanism and a sectional soot model. The results show that the high-temperature
region moves toward the wings of flame and the radius of the flame
decreases as ambient pressure increases. Soot volume fraction increases
significantly with its peak value scaled with p
2.25 for the pure methane flame and p
1.60 for the methane flame doped with n-heptane.
In addition, the height at which the initial soot forms moves upstream,
while the height at which soot particles are completely oxidized moves
downstream, resulting in a larger sooting region. The increase of
soot concentration is mainly due to the increase of the mixture density
as pressure changes, and therefore, the collision frequency between
gas species and particles increases, which in turn accelerates the
inception rate and surface growth rate of particles. Primary number
density also increases because of the increase in the inception rate.
The aggregate characteristics, which are evaluated by the number of
primary particles per aggregate, increase because of the increase
in the particle number density that leads to an acceleration in particle
coagulation. Soot mass addition, dominated by hydrogen-abstraction–carbon-addition
reactions, increases because of the increase of absolute concentrations
of C2H2 and H radicals, while soot mass consumption,
dominated by OH oxidation, increases with a lower rate than that of
soot addition because of the increase of the collision frequency and
efficiency between the OH radicals and particles.
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